Product package having reliable openability

ABSTRACT

A plastic packaging material which, when formed as a package having seams, will provide a predictable line of failure along a seam when the package is opened to prevent the package from tearing down the side. The predictable failure path is provided through a lamination process involving specific resins or blends of resins laminated in three (or more) layers in which an extruded inner layer forms a weak inner bond wholly within that inner layer to create the predictable line of failure and in which the process of forming the material does not inhibit processing speed, efficiency, and economics of materials used to provide this reliable openability.

This is a divisional of application Ser. No. 08/613,412 filed on Mar.11, 1997, now U.S. Pat. No. 5,689,935, issued on Nov. 25, 1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to product packaging and inparticular to a plastic material for forming a good product packagehaving a predictable line of failure when the package is opened toprevent tearing of the bag down the body of the bag. This predictablefailure path is provided through a coextruded lamination manufacturingprocess involving specific resins or blends or resins coextruded inthree or more layers that do not inhibit processing speed, efficiency,and economy of materials used to provide this reliable openability.

2. Description of Related Art Including Information Disclosed Under 37CFR 1.97 and 1.98

Laminated films produced for making food packages are based on multiplelayers of plastic film each with a specific purpose. An outer web isdesigned to move across a packaging machine, another web provides heatstability to prevent distortion when the package is sealed, stillanother provides the ability to obtain a uniform print surface, and yetanother provides the ability for the laminated film to be adhered to anadhesive used to bond the laminated film to an adjacent film. When morethan two film webs are used to form a laminated film, these core filmwebs provide additional specific properties such as moisture vaportransfer rate, gas barrier, and appearance. This part of the laminationhas nothing to do with the openability of the lamination in packageform. The inner web of a lamination can contribute barrier qualities,optics, and sealability, and yet provide openability that varies from"impossible" to "easy open".

Thus, a prior art bag so constructed is illustrated in FIG. 1 whereinthe bag 10 is shown filled with product and sealed. It has alongitudinal seal 12 and end seals 14 and 16. The bag is generally madeas indicated in FIG. 2 by forming essentially a cylindrical tube 18 witha longitudinal seal 12 and with a horizontal seal at the lower endthereof along seal 16 as shown in FIG. 3. The package can then be filledwith product when constructed as shown in FIG. 3 and sealed asillustrated in FIG. 4 to form a bag filled with product having upper andlower sealed ends 14 and 16, respectively, and a longitudinal side seal12.

The problem with these bags is illustrated in FIG. 5. When the bag is tobe opened and forces are applied in the direction of arrows 24 and 26 tothe upper seal 14, and bag may start to open at 26 and then teardownwardly into the side of the bag as illustrated at 28. This is not aserious problem for small bags of food products because the contents ofthe small bag can be eaten entirely. However, with large bags containinglarge amounts of food products, the tear 28 prevents the bag from beingreclosed to protect the contents therein; thus the contents must betransferred to another container that can be sealed to protect thefreshness of the product. The reason that such bags tear as shown inFIG. 5 is because of their construction and the interlaminar strength ateach interface of the lamination. A history and description of such bagconstruction is helpful.

Coated Films

Prior to extrusion lamination with a thermoplastic adhesive, adhesiveswere, and still are, based on a single or two-part adhesive which"cures" to a hard bond that is very difficult to separate. With thistype of adhesive, openability of a package had to be built into theinner film. The inner film's primary purpose is sealability in order toprovide integrity to the package. Openability was based on either acoating on the inner film that acted as the sealant or a coextrusionfilm or a thin sealant ply that separated from the inner film when thepackage was opened. While these methods did provide "openability", itwas inconsistent and varied with the temperature of the sealing jaws,sealant ply thickness when using a coextrusion film, and with theadhesion of the coating to the inner film due to primer or treatmentvariations. In addition, there was a narrow hot tack (ability to keepend seals together at the hot conditions necessary to obtain a seal) andseal range with coextruded inner films. The same problems occurred withcoated film if the film treatment and/or primer weight/drying was toolow.

Opp Films

Because of cost competition, the use of coextruded orientedpolypropylene films (OPP) began to replace coated films. These newfilms, which were and are typically two and three-layer coextrusions ofhomopolymer polypropylene as a core and copolymer/terpolymer skins, didnot, and do not, provide consistent openability of packages withouttearing of the package when it is opened. As the need for largerpackages with multiple usage has expanded, the failure of packages basedon laminations with oriented polypropylene coextrusions films as theinner web became unacceptable. Therefore, a solution was needed toprovide reliable openability of these larger packages along apredetermined path without tearing of the package down the inside whenit is opened.

With the change from coated sealant films or a dissimilar thermoplasticskin such as Surlyn® coextruded with polypropylene that has some degreeof openability due to built-in fracture or delamination lines to acoextruded polypropylene film that has poor openability, tearing thepackage is a serious problem. The increase in large packages withmultiple use needs has made tearing of the package when openingunacceptable.

Problems to Overcome

A process to produce a usable (opening without tearing) extrusion basedlamination between an inner sealant film and an adjacent outer film intwo-ply laminations, or the core film in a three-ply lamination mustprovide negligible loss in processing efficiency and yield loss and alsoenable the use of cost effective resin components to be commerciallyfeasible. For example, if an extrusion process typically runs at 1200ft/min, it is impractical to use a similar extrusion process that, whilemaking a usable product, runs at 300 ft/min. In addition, the ability toproduce product that has a flat profile across the web cannot becompromised by a product that meets end use requirements but has poorprofile and results in poor machine performance at the end user. Also,extrusion laminating of polypropylene films (typically one web isprinted) requires polypropylene films with surface treatment and/ormodification with other resins to provide a surface to whichpolyethylene can obtain adhesion. Polyethylene, when extruded, has anon-polar surface unless it is extruded at a sufficiently hightemperature and is exposed sufficiently to air through what is called anair gap (gap between the extrusion die lips and the extrusion nip) inorder to provide sites which can be bonded to similar sites onpolypropylene film surfaces. In addition, primers are typically used toprovide adhesive to the inks on the inner surface of one of the films tobe laminated. If any of the polyethylene extrusion parameters are not ata specific level, poor adhesion of the lamination will result. Thismanifests itself as a delamination at one or both of the filminterfaces. Because of the critical nature of the polyethylene extrusionprocess and the speed at which the lamination process occurs (typically1000-1500 ft/min) the bond strength at the inner surfaces of thepolypropylene films must be strong and flexible to obtain a level ofadhesion that provides usable handling during distribution of filledpackages.

Because of the need for reliable bond strength between the polypropylenefilms in a polyethylene lamination and the seal strengths of coextrudedpolypropylene films, it is difficult for a lamination using apolypropylene sealant film to provide consistent, reliable openabilitywithin the film layers or interfaces of the lamination.

Thus, consider the lamination of a prior art film as shown in FIG. 6.The lamination consists of outer polypropylene film layer 30 withtreated surface 31, ink layer 32, primer layer 34, if needed,polyethylene extrudable adhesive layer 36, a sealable polypropylene film38 with treated surface 39 and with copolymer or terpolymer sealant 40.It should be noted that outer layer 30 can be any acceptable plasticpackaging film and that inner layer 38 can be any sealable plasticpackaging film such as coex film, coated film, or metallized film.

Primer 34 is used to provide strong adhesion of the inks to the outerply of the extrudate 36. It also provides additional strength betweenthe treated polypropylene film surface 31 and the extrudate 36. A primerserves two main functions, i.e.,

1) It helps to provide an uncontaminated surface so that the extrudate36 will rapidly spread and uniformly web out.

2) The primer joins both the ink 32 and substrate 30 to the extrudate 36by a covalent bond, hydrogen bond, van der Waals force, dipoleinteraction, or some mixture of these effects. The ability of the primerto increase the number of reactive sites on the substrate and maximizeintermolecular attraction is important with non-polar surfaces such asinks, marginally treated surfaces, or treated surfaces that may havesome surface interference over the treated areas. There are inks andpolypropylene surfaces that do not require a primer for acceptable endproduct use but a primer is frequently used to maximize adhesion andprovide the ability to operate at higher laminator speeds. In the caseof the present invention, a primer may or may not be used, but in thepreferred product, it is used to maximize adhesion to the inks.

Continuing with FIG. 6, the sealable polypropylene film 38 isapproximately 0.70 mil and, when this package layer is sealed to theopposing package layer, the 0.70 mil sealant film 38 fuses to theadjacent 0.70 mil film and results in approximately 1.40 mils of filmforming the entire seal.

When the package is opened, the inner film fractures starting at a pointon the lower edge of the seal interface as shown at 42. The fracture canthen proceed through the 0.70 mil polypropylene film layer 38, throughthe polyethylene extrudable adhesive layer 36, the primer 34 (ifpresent), through the ink layer 32, and into the outer polypropylenefilm 30 which, of course, allows the bag to split down the side. Whenthe bond strength of the lamination is high enough to provide a usablelamination for normal distribution, the tendency is for the appliedforce shown by arrows 24 and 26 in FIG. 5 to break to the weakest pointand split down the lamination or through the total lamination because ofa lack of a consistent failure path within the lamination which, itself,has very low tear strength. Any irregularities in the seal such as afoldover can cause the splitting to occur much easier.

FIG. 7 illustrates a second prior art bag lamination structure with theink on the inside of the film. Again, with this construction, when thepackage is opened, the inner film 54 fractures starting at a point onthe lower edge of the seal interface as shown at 58. The fracture canthen proceed as indicated by arrow 60 through the 0.70 mil orientedpolypropylene inner film 54 and treated surface 55 to the ink, andthrough the ink layer 52 itself, into the polyethylene adhesive 58 andthrough the primer 50 (if present), into the outer polypropylene treatedinterface 47, and then into the outer polypropylene film 46 itself.Again, the interlaminar bond strength of the lamination is high enoughto provide a useful lamination for normal distribution, the tendency isfor the applied force, shown by the arrows 24 and 26 in FIG. 5, to breakto the weakest point and split down the lamination or through the totallamination because of the lack of a consistent failure path within thelamination which itself has a very low tear strength.

Because of this, it would be desirable to have a plastic product baghaving a seal that includes a predictable line of failure when thepackage is opened.

REQUIREMENTS OF THE INVENTION

The present invention discloses a food product package film having aseal to provide a predictable line of failure when a package is opened.This predictable failure path is provided through a coextrudedlamination manufacturing process involving specific resins or blends ofresins coextruded in three or more layers that do not inhibit processingspeed, efficiency, and economy of materials used to provide thisreliable openability.

In order to obtain strong adhesion, the inner and outer films must havetreatment on the inner surfaces facing the polyethylene outer plys ofthe three ply extrudate. Treatment of the laminating side of thepolypropylene film provides bonding sites to which similar bonding siteson the polyethylene outer plys of the extrudate can bond.

For example, the outer plys of a three layer extrudate based onpolyethylene/polypropylene/polyethylene, provide chemically reactivesites such as carbonyl and hydroxyl groups that will bond to similargroups on the polypropylene treated surface by a mixture of covalentbonds, hydrogen bonds, van der Waals forces, dipole interaction, or amixture of these effects. The reaction sites on the polyethylene outersurfaces are formed because of the temperature of extrusion (600°-620°F.) and oxygen in the air.

However, the inner plys of the extrudate, i.e., polyethylene topolypropylene, because they are not exposed to air have these chemicalbonding sites at a negligible level. The result is that bonding at theseinterfaces is weak and depends upon the flexible nature of the resinsused and the mechanical pressure of the lamination process to provideadhesion. This adhesion level with essentially untreated non-polarinterfaces is in the 20-50 gms/in range. This would typically beconsidered non-functional, but in this novel process, the combinedlamination with strong functional bonds at all other interfaces providesa finished product capable of withstanding all required finished productneeds.

1. The Performance Requirements

The resin or resins used to bond inner polypropylene sealant film andthe adjacent film (a core in a three-ply lamination and an outer film ina two-ply lamination) must provide inner laminar adhesion comparable towhat is attainable with extrudable polyethylene resin.

A core layer in a coextrusion process that provides a reliable failurepath and opening must be extrudable at normal operational speeds andmust provide final coextrusion gauge profile comparable to what isobtainable with a polyethylene resin extrudate.

A resin that provides the above predictable failure path in acoextrusion must be extrudable at a temperature that will allow it to becoextruded with a typical extrusion grade polyethylene resin, i.e.,0.917 density/3.5-18 melt index (MI). Finally, this resin must notimpart objectionable odor or color and must be stable to extrude.

With the understanding that, for a package to be opened, fracture of theinner sealant polypropylene film (oriented or cast) is the initiation offailure, the concept is to insure that a predictable failure mode occursat a specific interface such as the inner film/ink (when ink is at theinner film interface), within the ink at the inner film/extrudateinterface in unprinted areas, at the ink/primer/extrudate interface,within the extrudate, and at the extrudate to outer film interface.

2. Discussion of Potential Problems in Providing a Predictable FailureMode When a Package is Opened.

Because of the lack of flexibility of the inks used to print films,failure can and frequently does occur at the ink interface when opening.The same failure may occur within the ink itself. However, over 20 yearsof production experience demonstrates that this is not a consistentpredictable failure path. In order to compensate for the poor-to-fairadhesion of inks to a polyethylene extrudate, a primer is required.Normally this is a polyethyleneimine based primer. It has been shownthat the strength obtained at the ink/inner film and within the ink filmitself is too high to enable acceptable in-use performance such that areliable failure path will be provided. The result is that the forceapplied to open a package fractures the inner film and failure at theink/inner film or ink interface does not consistently occur because bondstrength is not consistent or reliably low.

Further, with the existing ink formulation/primers, it is impracticableto obtain adhesion to the ink interface with a polyethylene extrudatethat provides openability and handling resistance. Without a primer toprovide functional bonds of the ink to the polyethylene extrudate, thepackage will not withstand in-use handling. An alternative is to usecoextrudable adhesive copolymer resins or formulated resins that,because of their chemical nature, will adhere to the ink and filmportions of the inner film. However, the nature of these resins is thatthey are costly, require extrusion at temperatures low enough to preventdegradations of the adhesions promoting resins thus minimizing adhesionof the total extrudate to the outer web of the lamination, do notpromote good profile, and have odors that can be unacceptable for foodpackaging applications. In addition, most of these resins require longerdwell time, that is, they need pressure and time for maximum adhesion,and are thus not suited for extrusions at speeds in the 1000-1500 ft/minrange. Another disqualification is that, while they may have what isconsidered good adhesion to ink or films, they may not withstandhandling of the finished package due to a lack of flex resistance at theink interface.

Concerning the polyethylene extrudate, due to the flexible nature of theextrudate, it is impossible to make the failure occur within this ply ofthe lamination.

Further, it is not possible to insure that the failure mode will occurat the interface between the polyethylene extrudate and the outer filmbecause the bond strength of the polyethylene extrudate to the outerfilm in a lamination must be strong enough to insure handling ability inpackage form during distribution. Thus the mode of failure can occur atany or a combination of interfaces or through the whole lamination. Atany of these points the opening force exerted can cause tear propagationto occur at right angles to the seal as shown in FIGS. 6 and 7.

SUMMARY OF THE INVENTION

The present invention provides a predictable line or path of failurewhen the package is opened. This predictable failure path is providedthrough a manufacturing process involving specific resins or blends ofresins coextruded in at least three layers that does not inhibitmanufacturing processing speed, efficiency, and economy of materialsused to provide reliable openability.

A coextrusion extrudate has been developed that will providesubstantially consistent failure within the extrudate while at the sametime maintaining the economy and performance of polyethylene as theprimary extrudate in the lamination manufacturing process. It providesno change in machine performance during the packaging operation andprovides substantially consistent openability by the consumer.

The novel results are obtained by using a coextrusion based on apolyethylene/polypropylene/polyethylene extrudate.

The concept is not limited to the use of high pressure low densitypolyethylene (LDPE) as one or both of the extrudate layers encapsulatingthe polypropylene core. Typically the LDPE resins usable in this processhave a melt index from 2.0 to 35.0 and a density from 0.88 to 0.965. Apreferred polyethylene low density resin has a melt index from 3.5 to17.0 and densities from 0.914 to 0.926. A polyethylene resin may bemanufactured by either a high or low pressure polymerization process.

Linear low density polyethylene coating grades similar to Dow 3010 orQuantum GS-550 can be used in this process.

The process is not limited to homopolymer polypropylene core resinssince different degrees of openability can be obtained by usingcopolymer polypropylenes or blending homopolymer polypropylene withother polypropylenes or polyethylene as the core resin. The core canalso be based on nylon, PET, EVOH, and the like to arrive at the desiredopening strength.

An essential part of this concept is that the polyethylene can be one ormore of the outer plys of the extrudate. The dissimilar melttemperatures of polyethylene and polypropylene (or polypropylene resinand blends with specific other adhesion-enhancing resins) provide theclean mode of failure. The flexible nature of polyethylene provides thehandleability needed in the finished lamination for normal handling ofthe finished package.

It is possible to increase the bond strength between the core and theouter polyethylene extrudate layers by blending EMA resins (exampleChevron 2207) up to 35% by weight.

A significant part of this concept is that it be an extrusionlamination. By using this process it is possible to maintain the sealstrength and hot tack of the whole inner film and, when opening thepackage, drive the failure point to specifically occur at thecoextrusion laminate interface. In laminations using curing-typeadhesives, this concept is not possible because of the strong bonds thatoccur between the plys of the lamination.

It is possible to provide openability using a coextruded inner sealantfilm with specific failure points but the limitation in hot tack, sealrange, and economics of making the film are not as consistent andreliable as the concept set forth above.

The nature of easy-opening interlaminar failure provides for use ofsealant films not normally usable for each opening. For example: Castpolypropylene or films with unusual linear characteristics such as highdensity polyethylene or crystalline high-moisture barrier polypropylenefilm may be used.

Thus it is an object of the present invention to provide an easy-openingproduct plastic bag that generally does not tear down the side of thebag when a seam is opened.

It is another object of the present invention to provide a product bagformed of plastic that has a seal in which is provided a predictableline of failure when the package is opened.

It is still another object of the present invention to provide apredictable failure path in a good product package through a coextrudedlamination manufacturing process involving specific resins or blends orresins coextruded in three or more layers.

It is yet another object of the present invention to provide a goodproduct bag having a predictable line of failure in a seal and whichseal can be formed through coextrusion in a manner that does not inhibitprocessing speed, efficiency, or economics of material sued to providethis reliable openability.

Thus the present invention relates to a plastic material for forming aproduct package having a sealed seam along which a substantiallypredictable line of failure occurs when the package is opened. Theplastic material comprises first and second outer layers of plastic filmmaterial and a third extrudate layer of plastic material interposedbetween and bonded to the first and second outer layers, the thirdextrudate layer having characteristics such that a line of weakness isformed wholly within the extrudate that tears easier than the first andsecond outer layers and forms a substantially predictable line offailure wholly within the plastic extrudate material, the line offailure occurring during opening a package when a separating force isapplied to the sealed seam.

The invention also relates to a product package formed of plastic andhaving a substantially predictable line of failure along an elongatedseam that opens the package when forces are applied to the elongatedseam. The package comprises a plastic body portion having asubstantially enlarged portion for containing the product. Sealed edgesare formed on both ends of the body portion. The body portion is formedof a laminate using an extrudate adhesive layer laminated between aninner sealant plastic film layer and an outer plastic film layer. Asealant is formed on the inner surface of each end of the plastic bodyportion for joining two edges and closing each end of the package. Theextrudate may be formed of polyethylene/polypropylene/polyethylenehaving different melt temperatures for forming the predictable line offailure along the interface wholly within the extrudate between thepolypropylene and the polyethylene without tearing down the side of thebag.

The invention also relates to a process for forming a material used tocreate a product package having edges forming-a sealed seam along whicha substantially predictable line of failure occurs when force is appliedto the edges of the seam. The process comprises the steps of providingfirst and second outer layers of plastic material, and extruding a thirdlayer of plastic extrudate in an interposed relationship between thefirst and second outer layers of plastic material to form a multilayerlamination, the third layer having specific internal adhesioncharacteristics such that a line of weakness is formed wholly within theextrudant that tears easier than the first and second outer layers andforms a substantially predictable line of failure along a package seamformed by sealing edges of the plastic material, the line of failureopening the package when a separating force is applied to the sealededges without tearing the side of the bag under normal opening forcesbeing applied.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will be more fullydisclosed when taken in conjunction with the following DETAILEDDESCRIPTION OF THE PREFERRED EMBODIMENTS in which like numeralsrepresent like elements and in which:

FIG. 1 is a diagrammatical representation of a product package that issealed on both ends;

FIG. 2 is a schematic representation of a plastic film rolled to acylindrical form preparatory to forming a product bag;

FIG. 3 is a schematic representation of the lower end of the cylindricalfilm illustrated in FIG. 2 being sealed before product is place in thebag;

FIG. 4 is a schematic representation of the bag of FIG. 3 having producttherein and being sealed at the top portion;

FIG. 5 is a schematic representation of a prior art bag tearing down theside when forces are applied to the top seam to open the bag;

FIG. 6 is a representation of a prior art laminated package film whichis so constructed that a tear may occur through to the outside and downthe side of the bag;

FIG. 7 is a representation of a prior art film layer similar to that inFIG. 6 except that the ink is on the inside of a polyethylene film;

FIG. 8 is a schematic representation of the novel film of the presentinvention illustrating outer polypropylene layers separated by amultilayer extrudable adhesive composed ofpolyethylene/polypropylene/polyethylene to form a predictable line offailure wholly within the extrudate when the bag is opened;

FIG. 9 is a schematic representation similar to FIG. 8 except that theink is on the inside of the multilayer extrudable adhesive; and

FIGS. 10A and 10B are diagrammatic representations of the extrudationprocess of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 8 is a schematic representation of a packaging film of the presentinvention that will allow easy opening of a package by providing asubstantially consistent line of failure within the package seal. As canbe seen in FIG. 8, the film 62 includes an outer polypropylene filmlayer 64 and an inner sealable polypropylene film layer 66. An ink layer68 and primer layer 70 may be added as needed.

The novel ability to provide a substantially consistent failure line isprovided by the multilayer separately extrudable adhesive 72 thatincludes polyethylene layer 74, polypropylene layer 76, and polyethylenelayer 78. The dissimilar melt temperature of the polyethylene 74, 78 andthe core polypropylene 76 or polypropylene resin blends (with specificother adhesion enhancing resins such as EMA) provides the clean mode offailure wholly within the extrudate 72. Further, the flexible nature ofpolyethylene and polypropylene provides the handling ability needed inthe finished lamination for normal handling of the finished package.Thus a tear may begin at the point designated by the numeral 80 andspread into the inner layer 66 along any particular path such as paths82 or 86. When the tear following path 82 passes through thepolypropylene layer 78, the line of failure will tend to occur at theweak interface 84 between the polypropylene layer 76 and thepolyethylene layer 78. The interface 84 is weak because of the differentmelt temperatures of the polypropylene 76 and the polyethylene 78.Should, however, the tear proceed along a different path 86 and manageto extend through the polypropylene layer 76, it will encounter theinterface 88 between the polypropylene layer 76 and the polyethylenelayer 74 which, again, is a weak area along which the tear can occur atinterface 88. Again, the different melting temperatures of thepolyethylene 74 and the polypropylene 76 form the weak interface 88. Ineither case, the tendency will be for a tear to occur wholly within theextrudate 72 along a substantially consistent line of failure on eitherside of the polypropylene layer 76 and prevent the tear from proceedingthrough the polyethylene layer 74, the primer and ink layers 68 and 70,and the outer polypropylene film layer 64. Such, of course, prevents thebag from tearing down the side since the tear never reaches the externalsurface but follows the substantially predictable lines of failure alonginterfaces 84 and 88 wholly within the multilayer extrudate 72.

Thus FIG. 8 illustrates the film layer 62 that utilizes a coextrusionlamination having a polyethylene/polypropylene/polyethylene extrudatebetween an inner sealant film 66 of preferably oriented polypropylene orcast oriented polypropylene and an outer film 64, preferably apolypropylene, in a three-ply lamination that will provide consistentpackage openability at a specific failure point wholly confined betweenthe polyethylene and the polypropylene interfaces of the extrudate layer72.

It should be noted that the invention is not limited to two outer filmlayers separated by an extrudate layer but could include multiple layercombinations.

FIG. 9 is an alternate embodiment in which the multiple layer film 90comprises an outside polypropylene layer 92 and an inside sealablepolypropylene film 94 including sealant 96. The primer 97, is needed,and ink 98 may be placed on the sealable polypropylene film 94 as shown.The novel multilayer extrudable adhesive 100, comprising polyethylenelayer 102, polypropylene layer 104, and polyethylene layer 106 isextruded between the polypropylene film 92 and the primer 97. Again,when a tear occurs at a weak spot 108 along the sealable polypropylenefilm 94, it may generate along any one of several paths such as 110 and112 into the interior of the film layer 90. It may pass through the inklayer 98, the primer layer 97, and polyethylene layer 106 as shown byarrow 110. However, when it reaches the weak interface 114 between thepolypropylene layer 104 and polyethylene 106, it tends to propagatealong this interface because it is a specifically designed line offailure because of the different melt temperatures of the polypropylenelayer 104 and the polyethylene layer 106. If the tear does happen topropagate along the path 112 so that it extends through thepolypropylene layer 104, it will again strike a layer 116 of weaknessthat will cause the separation of the layers to occur along theinterface 116 which is the line of weakness or designed line of failure.Thus, again, the predictable line of failure lies wholly within theextrudate 100.

It is to be understood that different degrees of openability can beobtained by using a blend of polypropylene homopolymers and otherhomopolymer propylene or copolymer polypropylenes, and polyethylene, orother blends of these resins may be formed as the core in thepolyethylene/polypropyleneblend/polyethylene coextrusion therebycontrolling openability of a finished package in a coextrusionlamination.

It should also be understood that this concept is not limited to the useof high pressure LDPE as one or both of the extrudate layersencapsulating the polypropylene core. Typically the LDPE resins usablein this process have a melt index from 2.0 to 35.0 and a density from0.88 to 0.965. The preferred polyethylene low-density resins have a meltindex from 3.5 to 17.0 and densities from 0.914 to 0.926. Thepolyethylene resins may be manufactured by either a high or low-pressurepolymerization process. Linear low-density polyethylene coating gradessimilar to Dow 3010 or Quantum GS-550 can be used in this process.

The use of ethylene methyl acrylate copolymers such as Chevron 2207 canbe blended into the polyethylene outer ply for selective adhesion to theinner surfaces of the lamination. The EMA copolymer can be blended up to35% by weight into the polyethylene resin used as the outer plys of thecoextrusion.

One of the polyethylene skins or layers can be based on a differentpolyethylene resin or blend to enhance the overall strength of thelaminate, i.e., toughness, puncture, and the like.

FIG. 10A is a schematic representation of the extrusion process by whichthe polyethylene/polypropylene/polyethylene extrudate adhesive novel plyin FIG. 8 or in FIG. 9 is formed. The actual resin extruding devices arewell known in the art and typically are based on a combining adapter/die131 where the extrudate melts are combined into one extrudate withspecific layers. Thus as can be seen in FIG. 10A, the novel film layer62, is composed of two outer polyethylene extrudates 130 and 132 and aninner polypropylene core 138 all of which are extruded and thenlaminated between the laminating rollers 119.

The extrudate 62 is separately formed of the polyethylene layer 130 andpolyethylene layer 132 coming from common (shown) or separate dieextruders. The polypropylene layer 138 is coextruded from one extruder(not shown) and is between polyethylene layers 130 and 132 which areproduced by another extruder (not shown). The extrudate 62 is thenlaminated with the polypropylene layers 120 and 122 through laminationrollers 119.

As can be seen in FIG. 10B, other polyethylene resins or blends 140 canbe used as one of the polyethylene layers for selected adhesion and forenhancement of the physical properties of the finished lamination suchas for instance, a barrier layer, puncture resistance and the like.

The use of ethylene methyl acrylate copolymers such as Chevron 2207 canbe blended into one or more of the polyethylene outer plys for selectiveadhesion to the inner surfaces of the lamination. The ethylene methylacrylate copolymer can be blended up to 35% by weight into thepolyethylene resin used as the outer plys of the core.

The results of a first test -series using the novel film is shown inTable 1. It compares package openability without tearing of both priorart laminations and the novel laminated product. The basis of the testis the openability of 100 packages of each test group using typicalopening force. The outer film, inner film, and inks are the same on thePrior Art and Novel laminations. The difference between prior art testmaterial I and II is a target of 10 and 7 #/ream total extrudate weightand the gauge of the outer and inner films. The total extrudate weightin the prior art samples is polyethylene. In tests I and II of the novelmaterial of the present invention, a three-layer coextrusion of lowdensity polyethylene is used as the two outer plys with a core ofpolypropylene in the novel samples. It will be seen that 83 and 87 bagsof the prior art tore down the side of the bag while with the presentinvention, only 2 and 3 bags out of the 100 samples tore down the sideof the bag.

                                      TABLE I                                     __________________________________________________________________________    TYPE     PRIOR ART                                                                            PRIOR ART                                                                            NOVEL  NOVEL                                           STRUCTURE                                                                              I      II     I      II                                              __________________________________________________________________________    Polypropylene                                                                          75     100    75     100                                             Outer Film                                                                    (Gauge)                                                                       Extrudate                                                                     Outer Layer                                                                            10 #/Ream                                                                            7 #/Ream                                                                             4.0 #/Ream                                                                           2.5 #/Ream                                               Total  Total  Polyethylene                                                                         Polyethylene                                             Polyethylene                                                                         Polyethylene                                                  Core     10 #/Ream                                                                            7 #/Ream                                                                             2.0 #/Ream                                                                           2.0 #/Ream                                               Total  Total  Polypropylene                                                                        Polypropylene                                            Polyethylene                                                                         Polyethylene                                                  Outer Layer                                                                            10 #/Ream                                                                            7 #/Ream                                                                             4.0 #/Ream                                                                           4.0 #/Ream                                               Total  Total  Polyethylene                                                                         Polyethylene                                             Polyethylene                                                                         Polyethylene                                                  Primer   Yes    Yes    Yes    Yes                                             Ink      Ink    Ink    Ink    Ink                                             Inner Polypropylene                                                                    70     120    70     120                                             Film (Gauge)                                                                  # of Packages                                                                          83     87      2      3                                              Tearing                                                                       # of Packages Not                                                                      17     13     98      97                                             Tearing                                                                       % of Openability                                                                          17%    13%    98%     97%                                         w/o Tearing                                                                   __________________________________________________________________________

Concerning the novel I and II structures of the present invention shownin Table I, two pounds/ream is the preferred target polypropylene coreweight but weights as low as 1.0 #/ream are acceptable. There is nolimit on the upper polypropylene weight as long as a continuouspolyethylene layer is present on each side of the polypropylene. Thedistribution of the outer polyethylene layer does not have to besymmetrical, but it is critical for consistent package durability that acontinuous polyethylene layer be present on each side of thepolypropylene cord. For example, apolyethylene/polypropylene/polyethylene distribution in #/ream of1.0/2.0/9.0, 9.0/2.0/1.0, 3.0/2.0/7.0 will all provide acceptableresults. If the polyethylene is absent from the side of the extrudatefacing the print, the package handling resistance is not acceptable.

Table II illustrates the results of a second test series in whichcomparison of the openability of prior art laminations based on filmswith tear resistance poorer than those used with the normal prior artproduct are compared to the same laminations utilizing the novelextrusion lamination process of the present invention. Percentopenability is based on 100 packages tested.

                                      TABLE II                                    __________________________________________________________________________    TYPE STRUCTURE                                                                             PRIOR ART I                                                                           PRIOR ART II                                                                          PRIOR ART III                                                                         NOVEL I  NOVEL II                                                                              NOVEL                   __________________________________________________________________________                                                          III                     Outer Film   80 Ga. High                                                                           75 OPP  75 OPP  80 Ga. High                                                                            75 OPP  75 OPP                  (Gauge)      Barrier OPP             Barrier OPP                              Total Extrudate                                                                            10 #/ream                                                                             10 #/ream                                                                             10 #/ream                                                                             10 #/ream                                                                              10 #/ream                                                                             10 #/ream               Outer        Polyethylene                                                                          Polyethylene                                                                          Polyethylene                                                                          4 Polyethylene                                                                         4 Polyethylene                                                                        4 Polyethylene          Core         Polyethylene                                                                          Polyethylene                                                                          Polyethylene                                                                          2 Polypropylene                                                                        2 Polypropylene                                                                       2 Polypropylene         Inner        Polyethylene                                                                          Polyethylene                                                                          Polyethylene                                                                          4 Polyethylene                                                                         4 Polyethylene                                                                        4 Polyethylene          Primer       Yes     Yes     Yes     Yes      Yes     Yes                     Ink          Ink     Ink     Ink     Ink      Ink     Ink                     Inner Film   70 Sealable OPP                                                                       80 Ga. High                                                                           1.0 mil 70 Sealable OPP                                                                        80 Ga. High                                                                           1.0 mil                 (Gauge)              Barrier Sealable Cast    Barrier Sealable Cast                                Sealable OPP                                                                          Polypropylene    Sealable OPP                                                                          Polypropylene           # of Packages Tearing                                                                      99      100     92      2        2       4                       # of Packages Not Tearing                                                                  1       0       8       98       98      96                      % Openability w/o Tearing                                                                  1%      0%      8%      98%      98%     96%                     __________________________________________________________________________

Note that when the standard 80 gauge high barrier film is used as theouter or inner layer in the prior art processes I and II, extremely poorpackage openability was obtained because of the fragile nature of thehigh barrier film. Thus, 99 and 100 of the 100 bags in each test toredown the side when opened. Table II shows in novel processes I and IIthat the same film can be used with the present novel process with anextremely high degree of success. As can be seen, only 2 and 2 of the100 bags in each test tore down the side. Thus, the novel process allowsthe use of standard films that normally provide poor performance in bagopenability. The prior art process III uses an inner film that make bagopening extremely difficult due to high seal strength. In that test, 92of the 100 bags tested tore down the side of the bag. Novel process IIIof the present invention uses the same inner film but provides a line ofweakness wholly within the extrudate layer that enables easy opening ofthe bags without tearing the body of the bag. As can be seen, only 4 ofthe 100 bags tested tore down the side.

Table III shows the results of a third test series that demonstrates theeffect of polypropylene core weight on openability and sets forth theweight of polypropylene in the total extrudate. Fifty packages weretested for openability on each test series. Note that excellent resultswere obtained over a wide range of such weights. However, the worstresults were obtained with a polypropylene core weight below 1.0.

                                      TABLE III                                   __________________________________________________________________________    Type Structure                                                                             1    2    3    4    5    6    7    8    9    10                  __________________________________________________________________________    Outer Polypropylene                                                                        75 OPP                                                                             75 OPP                                                                             75 OPP                                                                             75 OPP                                                                             75 OPP                                                                             75 OPP                                                                             75 OPP                                                                             75 OPP                                                                             75                                                                                 75 OPP              Film (Mils)                                                                   Extrudate    10 # Total                                                                         10 # Total                                                                         10 # Total                                                                         10 # Total                                                                         10 # Total                                                                         7 #/Ream                                                                           7 #/Ream                                                                           7 #/Ream                                                                           7 #/Ream                                                                           7 #/Ream            Polyethylene Outer                                                                         --   --   --   --   --   --   --   --   --   --                  Polypropylene Core                                                                         0.5-0.7                                                                            1.0-1.6                                                                            2.0-3.0                                                                            4.0-6.5                                                                            7.0-8.5                                                                            0.5-0.9                                                                            1.0-1.6                                                                            2.0-3.0                                                                            4.0-7.0                                                                            8                   Polyethylene Outer                                                                         --   --   --   --   --   --   --   --   --   --                  Primer       Yes  Yes  Yes  Yes  Yes  Yes  Yes  Yes  Yes  Yes                 Ink          --   --   --   --   --   --   --   --   --   --                  Inner Polypropylene                                                                        70   70   70   70   70   70   70   70   70   70                  Film (Mils)                                                                   # of Packages Tearing                                                                      10   3    1    0    0    11   2    0    0    1                   # of Packages Not Tearing                                                                  40   47   49   50   50   39   49   50   50   50                  % Openability                                                                              80%  94%  98%  100% 100% 78%  96%  100% 100% 98%                 __________________________________________________________________________

While the invention has been described in connection with a preferredembodiment, it is not intended to limit the scope of the invention tothe particular form set forth, but, on the contrary, it is intended tocover such alternatives, modifications, and equivalents as may beincluded within the sphere and scope of the invention as defined by theappended claims.

We claim:
 1. A process for forming a material used to create a productpackage having edges forming a sealed seam along which a substantiallypredictable line of failure occurs when force is applied to said edgesof said seam, the process comprising the steps of:providing first andsecond outer layers of plastic film material; and extruding a thirdlayer of extrudate in an interposed relationship between said first andsecond outer layers of plastic film material to form a multilayerlamination; said third layer having specific internal adhesioncharacteristics such that a weakness is formed wholly within theextrudate that tears easier than the first and second outer layers andforms a substantially predictable line of failure along a package seamformed by sealing edges of said plastic material, said line of failureopening said package when a separating force is applied to the sealededges without tearing down the side of the package.
 2. A process as inclaim 1 wherein the step of providing said first and second outer layersof plastic film material comprises the step of providing polypropyleneas said first and second outer layers.
 3. A process as in claim 2wherein the step of extruding said third layer of extrudate furtherincludes the steps of:extruding at least first and second layers ofpolyethylene separated by and bonded to at least one of polypropylene;selecting said polyethylene to have a first melt temperature; andselecting said separating layer of polypropylene having a melttemperature different from the polyethylene first melt temperature tocause a weak flexible interface between the polypropylene to thepolyethylene to form said line of failure.
 4. A process as in claim 3further comprising the steps of:selecting said polyethylene for saidfirst and second layers to have a melt temperature in the range of about212° F. to about 270° F.; and selecting said separating polypropylenelayer to have a melt temperature in the range of about 300° F. to about340° F.